A total of 17 different delta-thal alleles have been reported; most result in the absence of any delta chain synthesis but some should be considered delta+-thal variants. Their presence has no clinical consequences; however, when such a delta-globin gene mutant is observed, either in cis or in trans to a beta-thal allele, complications in the diagnosis of the beta-thal heterogeneity may occur because of a normal Hb A2 level of 2.0-2.5%. The 17 alleles are described in detail on pages 174 through 184; they are listed from 5' to 3' according to the locations of the changes. The sequence of the delta-globin gene is given in Fig. 8. The delta-thal alleles can also be listed according to the functional changes caused by the mutation or deletion as is shown in Table XVI.
FIG. 8. Sequence-Reading Chart for the Delta-Globin Gene